(a) Field of the Invention
The present invention relates to a thin film transistor and a liquid crystal display.
(b) Description of Related Art
A liquid crystal display (“LCD”) includes upper and lower panels provided with electrodes thereon and liquid crystal material interposed therebetween. The LCD displays an image by applying electric field to liquid crystal material interposed between the two panels using the electrodes and controlling the intensity of the electric field to adjust the transmittance of light passing through the panels.
The most popular one among those LCDs is the one that a common electrode and a plurality of pixel electrodes are formed on the respective panels, and a plurality of thin film transistors (“TFTs”) switching the voltages applied to the pixel electrodes are formed on the panel with the pixel electrode.
The most conventional TFT used in an LCD is an amorphous silicon TFT using amorphous silicon as semiconductor pattern.
The amorphous silicon TFT has electrical charge mobility of about 0.5–1.0 cm2/V·sec, and, therefore, it can be used as a switching element of an LCD. However, it is not proper to use the amorphous silicon TFTs for a driving circuit directly on the liquid crystal panel due to its insufficient electrical charge mobility.
To overcome this problem, a polycrystalline silicon TFT using polycrystalline silicon having electrical charge mobility of about 20–150 cm2/V·sec as semiconductor pattern is developed. Since the polycrystalline silicon TFT has relatively high electrical charge mobility as described above, Chip In Glass structure in which driving circuits are embedded in the liquid crystal panel can be implemented.
Techniques for obtaining polycrystalline silicon thin film include as-deposition technique depositing polycrystalline silicon directly on a substrate at high temperature, a solid phase crystallization technique depositing amorphous silicon and crystallizing at high temperature, a technique depositing amorphous silicon and crystallizing by laser, and so forth. However, since those techniques require a high temperature process, it is not proper for application of glass substrates for LCDs. Also, they have a disadvantage that electrical characteristics are not uniform between TFTs due to non-uniform grain boundaries.
To resolve these problems, a sequential lateral solidification technique, which can artificially control distribution of a grain boundary, is suggested. This technique uses the fact that the grain of polycrystalline silicon grows in a direction perpendicular to the boundary plane between a liquid phase region exposed to laser beam and a solid phase region which was not exposed to laser beam.
In the sequential lateral solidification technique, a laser beam passes through a transmission area of a mask having a slit pattern to completely melt amorphous silicon to form liquid phase regions arranged in a shape of the slit pattern in the amorphous silicon layer. Then, the liquid phase amorphous silicon becomes cooled to be crystallized. At this time, a grain grows from the boundary of a solid phase region which was not exposed to laser in a direction perpendicular to the boundary plane, and the grains stop growing when they meet at the center of the liquid phase region. Such sequential lateral solidification can crystallize the whole thin film by moving the slit pattern of a mask along the growing direction of the grains.
However, if the sequential lateral solidification process is performed by moving the slit pattern of the mask only along the above grain growing direction, the grains grow to several microns in the above grain growing direction, but they grow just some thousands of Å in a direction perpendicular to the above grain growing direction.
If the size of grain has anisotropy, electrical characteristics of TFTs formed on a substrate also have anisotropy depending on the channel directions. That is, electrical charge mobility has a large variation between directions parallel and perpendicular to the above grain growing direction, and this causes a design difficulty that all TFTs should be arranged in the same direction when TFTs are formed on the liquid crystal panel.
Generally, a data driver circuit and a gate driver circuit incorporated in a liquid crystal panel are arranged to be perpendicular to each other, and, even for data driver circuits, TFTs arranged in both transverse and longitudinal directions are required as the circuit becomes complicated. In this case, the above-described sequential lateral solidification may have a big disadvantage.
Therefore, if an amorphous silicon thin film is crystallized by the sequential lateral solidification technique such that anisotropy of crystallization characteristic is caused, driving circuit design becomes difficult and the size of the driving circuit becomes large due to the complicated wiring.